Cardiac pacer with switching circuit for isolation

ABSTRACT

A cardiac pacer uses switched capacitor circuits for maintaining isolation between an atrial channel subsystem and a ventricular channel subsystem. Cross-talk between the channels is prevented by the switched capacitor circuits which may include two sampling switches, a sampling capacitor, two transfer switches, and a filter. The switched capacitor circuits are disposed in series between the corresponding channel lead and the corresponding sensing circuit for that channel. An arrangement of switches and a corresponding controlling arrangement are used to change the mode of sensing and pacing of each channel independent of the mode used by the other channel. That is, each channel subsystem is independently operable for bipolar sensing, bipolar pacing, unipolar sensing, and unipolar pacing.

BACKGROUND OF THE INVENTION

This invention relates to cardiac pacers. More specifically, thisinvention relates to programmable cardiac pacers using isolationcircuits to avoid cross-talk.

The heart serves as the pump which forces the blood through the bloodvessels to all organs of the body. It is divided into four chambers, thetwo atria and the two ventricles. The atria serve as entryways to theventricles and pump weakly to help move blood into the ventricles. Theventricles are the main pumps forcing blood to the rest of the body. Foroptimum cardiac output, the heart must contract in a coordinated mannerwith the atria contracting first and the ventricles soon afterwards.

The heart has an intrinsic pacemaker, located in the right atrium, whichsets the heart rate and an intrinsic conduction system which maintainsthe required coordination among the chambers and among the individualmuscle fibers of the heart. When the intrinsic pacemaker or theconduction system operates improperly, a man-made pacemaker may beimplanted to take over the impaired functions.

A single chamber pacer may be used to stimulate either the right atriumor right ventricle. If the atrium is paced, the internal conductionsystem of the heart is depended on to carry the stimulus to theventricle. If the conduction system between the atria and ventricles isnot intact, the ventricle must be stimulated directly. An additionallead can then be placed in the atrium to sense normal atrial activity sothat the pacer can stimulate the ventricle in synchrony with the atrialactivity. If there is inadequate spontaneous atrial activity, the pacercan take over the function of the intrinsic pacemaker and stimulate theatrium followed by the ventricle after an appropriate delay.

Cardiac pacers may be classified as unipolar or bipolar, depending uponthe configuration of the common electrode. A unipolar pacer uses the canor housing of the pacer as the common electrode and the tip of the leadas the other electrode. Since the myocardium is generally paced with anegative going pulse, the can serves as a positive ground. Duringsensing, the measured potential is between the tip of the lead and thecan. A bipolar pacer has two electrodes on the lead, one at the tip andthe other a short distance proximal. During pacing, the tip of the leadis the negative terminal and the proximal electrode is the positiveelectrode of the pulse generator. During sensing, the potentialdifference between the two electrodes on the lead is monitored. Thus, inbipolar sensing or pacing, the can is not part of the electricalcircuit.

There are certain advantages to the bipolar configuration. Duringsensing, the system is less susceptable to environmental and muscleelectrical noise pickup. During pacing, the bipolar configuration isless likely to cause unwanted skeletal muscle stimulation than theunipolar configuration. There are, however, also drawbacks to thebipolar configuration. The sensed signals are somewhat more variablethan with unipolar sensing. Pacing is less energy efficient with bipolarstimulation and it is more difficult to detect the pacing artifact on asurface electrocardiogram.

It should be noted that a pacer may pace in one mode (unipolar orbipolar) and sense in the other mode.

Cardiac pacers having both atrial and ventricular channels are in commonuse. Usually each of the channels includes an associated lead whichcarries cardiac signals from the cardiac chamber to the channel sensingcircuit. Additionally, each channel subsystem includes a stimulatorwhich uses the lead for application of the stimulating pulse to thecardiac chamber, as necessary. However, in certain dual chamber pacingmodes, the sense amplifier or stimulator may be absent or non-functionalin either channel.

Cross-talk has posed significant problems in the operation of suchpacers when the two channels are not effectively isolated. Specifically,the application of a stimulating pulse by the pulse generator by way ofthe two electrodes at the end of a chamber lead may inadvertantlystimulate the other chamber due to return current in the other chamberlead and the resulting polarization of the electrode. Depending on themagnitude of the polarization, the other chamber may be stimulated andthe desired rhythm of the heart could be disrupted. In addition to suchcross-stimulation, the polarization of an electrode due to other channelpacing may be sensed as activity in the chamber served by the polarizedelectrode. Because demand type pacers rely upon their sensing circuitsto determine when a stimulating pulse should be applied, suchcross-sensing should be avoided.

One disadvantage of various prior art techniques is that the operationof one channel will be dependent upon the operation of the otherchannel. For example, prior art switching circuits to isolate thechannels may require that the sensing of the atrial cardiac signals stopwhen the ventricular stimulator is applying a stimulating pulse. Thisundesirable interdependence between the channels also commonly leads tocomplexity of design. More generally such complexity is a disadvantageof numerous prior art pacers including some lacking thisinterdependence.

Prior art systems using resistors alone or with other components toprevent cross-talk are disadvantageous in that high values of resistanceintroduce significant thermal noise into a circuit. However, high valuesof resistance may be necessary to provide the required isolation. Suchhigh value resistors often diminish the signal magnitude of the sensedcardiac signals, effectively decreasing the ability of the system tosense cardiac signals. This and other factors constrain the selection ofvalues for components such that such prior art systems may, ofnecessity, allow at least some cross-talk.

Although prior pacers have provided for some degree of programmability,the progammability of the pacers has been generally quite limited. Forexample, one prior art design requires that both channels (atrial andventricular) be operated in the same mode with respect to the type ofpacing (unipolar or bipolar) and the type of sensing. Other priordesigns have limited one to programming the pacing, but not the sensingor alternately programming the sensing, but not the pacing.

One factor in limiting the programmability of the prior cardiac pacerdesigns has been the problem of cross-talk between the atrial subsystemand the ventricular subsystem. That is, use of a particular pacing modeon one channel may cause such significant cross-talk as to preclude useof a particular mode of sensing on the other channel.

A further disadvantage of numerous cardiac pacer designs is thatcross-talk is prevented by turning off the sensing circuit of onesubsystem (either atrial or ventricular) when the other subsystem ispacing. This results in a loss of information to the sensor which isturned off.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea new and improved cardiac pacer, expecially one adapted forimplantation in a human patient.

A more specific object of the present invention is to provide a cardiacpacer wherein either of the atrial or ventricular channel subsystems maycontinue sensing cardiac signals even when the other channel subsystemis applying a stimulating pulse.

Another object of the present invention is to provide a cardiac pacerwherein cross-talk is prevented with little or no attenuation of sensedcardiac signals.

Yet another object of the present invention is to provide a cardiacpacer wherein an isolation circuit used to prevent cross-talk does notintroduce thermal noise.

A further object of the present invention is to provide a cardiac pacerusing a relatively simple and inexpensive design to avoid cross-talk.

A still further object of the present invention is to provide a cardiacpacer which is highly programmable to allow flexibility in operation.

The above and other objects of the present invention which will becomemore apparent as the description proceeds are realized by a cardiacpacer comprising a ventricular channel subsystem and an atrial channelsubsystem, each subsystem having a correseponding sensing circuit forsensing cardiac signals, at least a first of the subsystems having astimulator for applying cardiac stimulating pulses, and the second ofthe subsystems having a switched capacitor circuit operable to transmitsignals to the corresponding sensing circuit such that the secondsubsystem provides no return current path for stimulating pulses fromthe stimulator. The switched capacitor circuit includes a samplingcapacitor having first and second nodes and operable to sample cardiacsignals, controlled first and second sampling switches connectedrespectively to the first and second nodes of the sampling capacitor andoperable for switchably applying cardiac signals across the samplingcapacitor, a filter having first and second nodes and operable to storecardiac signals and to supply cardiac signals to the correspondingsensing circuit, and controlled first and second transfer switches, thefirst transfer switch switchably connecting the first node of thesampling capacitor to the first node of the filter, the second transferswitch switchably connecting the second node of the sampling capacitorto the second node of the filter. The pacer further comprises a samplingcontroller operable for controlling the first and second samplingswitches and the first and second transfer switches such that the firstand second sampling switches are both open whenever either of the firstand second transfer switches is closed and such that the first andsecond transfer switches are both open whenever either one of the firstand second sampling switches is closed. The first subsystem furthercomprises a bipolar pacing-sensing lead having first and secondconducting wires, and controlled first and second power switchesswitchable to allow power to flow from the the stimulator to the lead.The first subsystem is disposable in a sensing state and the controlleris operable to change the first subsystem between the sensing state anda pacing state in which the first and second power switches are closedto allow power flow from the stimulator to the lead. The first subsystemfurther includes a dissipation switch for shorting the first and secondconducting wires together, and the controller closes the dissipationswitch after the pacing state and before the sensing state. Thecontroller changes the state of both the first and second samplingswitches simultaneously and changes the state of both of the first andsecond transfer switches simultaneously. The second subsystem has astimulator for applying cardiac stimulating pulses. The first subsystemhas a switched capacitor circuit operable and including like componentsas with the switched capacitor circuit of the second subsystem. Thecontroller is operable to dispose each of the subsystems in thefollowing modes:

I. unipolar pacing and unipolar sensing,

II. unipolar pacing and bipolar sensing,

III. bipolar pacing and unipolar sensing, and

IV. bipolar pacing and bipolar sensing.

The controller is operable to set the mode of each subsystem independentof the mode of the other subsystem. The controller maintains the firstand second sampling switches of the subsystem open when the stimulatorof that subsystem is supplying cardiac stimulating pulses. Thecontroller allows, in at least some modes, the sensing of either one ofthe subsytems to continue sensing cardiac signals even when thestimulator of the other of the subsystems is supplying cardiacstimulating pulses and with the switched capacitor circuit of the onesubsystem preventing the stimulating pulses of the stimulator of theother subsystem from finding a return current path in the one subsystem.

The present invention may alternately be described as a cardiac pacercomprising a ventricular channel subsystem and an atrial channelsubsystem, each subsystem having: a lead for receiving cardiac signals;a sensing circuit for sensing cardiac signals; a stimulator forsupplying cardiac stimulating pulses; and a switched capacitor circuitoperable to transmit cardiac signals from the lead of the subsystem tothe sensing circuit of the subsystem such that the subsystem provides noreturn current path for stimulating pulses from the stimulator of theother subsystem. The pacer further comprises a controller forcontrolling each of the switched capacitor circuits. The controller isoperable to allow the sensing circuit of either one of the subsystems tocontinue sensing cardiac signals even when the stimulator of the othersubsystem is supplying stimulating pulses and with the switchedcapacitor circuit of the one subsystem preventing the stimulating pulsesof the stimulator of the other subsystem from finding a return currentpath in the one subsystem. The controller is operable to control firstand second sampling switches in the switched capacitor circuits of eachsubsystem and to control first and second transfer switches in theswitched capacitor circuit of each subsystem such that the first andsecond sampling switches of a subsystem are both open whenever either ofthe first and second transfer switches of that subsystem are closed andsuch that the first and second transfer switches of a subsystem are bothopen whenever either of the first and second sampling switches of thatsubsystem are closed.

The present invention may alternately be described as a cardiac pacercomprising: a housing; a controller; a ventricular channel subsystem;and an atrial channel subsystem. Each subsystem includes: twopacing-sensing conductors; a stimulator connected to the twopacing-sensing conductors for supplying cardiac stimulating pulses; anisolation circuit connected to the two pacing-sensing conductors; and asensing circuit connected to the isolation circuit for receiving sensedcardiac signals from the two pacing-sensing conductors by way of theisolation circuit. The isolation circuit of each subsystem isolates thesensing circuit of that subsystem against return current fromstimulating pulses from the stimulator of the other subsystem. The pacerfurther comprises a controller operable to dispose each of thesubsystems in the following modes, independent of the mode of the othersubsystem:

I. unipolar pacing and unipolar sensing;

II. unipolar pacing and bipolar sensing;

III. bipolar pacing and unipolar sensing;

IV. bipolar pacing and bipolar sensing.

In at least some modes, the isolation circuit of either one of thesubsystems allows the sensing circuit of the one subsystem to continuesensing cardiac signals even when the stimulator of the other of thesubsystems is supplying stimulating pulses and with the isolationcircuit of the one subsystem preventing the stimulating pulses of thestimulator of the other subsystem then finding a return current path inthe one subsystem. In at least some modes, the sensitivity of thesensing circuit in each subsystem is independent of the occurrence ofstimulating pulses from the stimulator of the other subsystem. The "atleast some modes" includes all modes except where the one subsystem isusing unipolar sensing and the other subsystem is using unipolar pacing.Each of the isolation circuits is a switched capacitor circuit includingthe elements as noted above. The controller includes a phase clockgenerator with non-overlapping first and second outputs, the firstoutput controlling the first and second sampling switches in both of thesubsystems and the second output controlling the first and secondtransfer switches in both of the subsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will be moreapparent when the following detailed description is considered inconjunction with the accompanying drawings wherein like charactersrepresent like parts throughout and in which:

FIG. 1 shows a simplified schematic of a programmable pacer according tothe present invention.

FIG. 2 shows a controller arrangement according to the present inventionand used for operating the controlled switches of FIG. 1.

FIG. 3 shows a simplified schematic of a bipolar cardiac pacer accordingto the present invention.

FIG. 4 shows a controller circuit arrangement for use with thecontrolled switches shown in FIG. 3.

FIG. 5 shows a filter construction for use with the present invention.

FIG. 6 shows an arrangement for programming the controller of thepresent invention.

FIG. 7 shows a phase clock generator as used with the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a simplified schematic of the cardiac pacer 50 of thepresent invention. The pacer 50 includes a controller 18 powered frombattery 19 which has its positive node connected to ground andswitchably connected to the pacemaker can or housing (can illustrated bychasis ground symbol) by way of a controlled switch 20.

As used herein, the words "switch" and "switches" shall be considered tocover switching elements of any kind including solid states switches andelectromechanical switches such as relays. For example, an MOS or othertype of FET may be used to realize a "switch" if its gate is controlledto alternately close and open the signal path between its source anddrain. A "controlled switch" as used herein means a switch having atleast three terminals, one of which gates or controls signal flowbetween the other two.

All of the switches shown in FIG. 1 are controlled switches. However,the control lines for these switches, such as switch 20, are not shownin FIG. 1 in order to more clearly illustrate the concepts of thepresent invention.

The battery 19 supplies energy to an atrial channel subsystem 52A and aventricular channel subsystem 52V. The controller 18 supplies aplurality of signals to the atrial channel subsystem 52A on lines 16Aand supplies a plurality of signals to the ventricular channel subsystem52V on lines 16V. The subsystems in turn supply a plurality of signalsto the controller 18 on lines 17A and 17V.

As the atrial and ventricular channel subsystems are of almost identicalconstruction, the components of the atrial channel subsystem 52A will bediscussed in detail with the understanding that the correspondingventricular channel components operate in the same fashion except asnoted otherwise. The corresponding components of the differentsubsystems 52A and 52V are noted with the same numerals with an "A"representing the atrial component and a "V" representing thecorresponding ventricular component.

The atrial channel subsystem 52A includes a bipolar pacing-sensingatrial lead comprising first and second conducting wires 12A1 and 12A2.A charging circuit 9A is used to charge an atrial stimulator capacitor10A. Controlled first and second atrial power switches 1A and 2Aswitchably connect the nodes of the capacitor 10A to the first andsecond conductors or conductive wires 12A1 and 12A2 except that a DCblocking capacitor 11A may be used between switch 1A and line 12A1.

A controlled atrial dissipation switch 3A is connected as shown in orderto discharge voltage which might otherwise remain on the lead due toparasitic capacitance on the bloodmetal interface. An atrial unipolardissipation controlled switch 4A is used to discharge such voltages whenthe atrial channel subsystem 52A is in a unipolar pacing mode as will bediscussed in detail below. Controlled switches 5A and 6A are used toalternately set the atrial channel subsystem 52A for unipolar sensing(6A closed and 5A open) or bipolar sensing (5A closed and 6A open).

An isolation switched capacitor circuit 54A is used to transmit cardiacsignals from lines 12A1 and 12A2 (bipolar sensing mode) or line 12A1 andthe pacemaker can by way of switch 6A being closed (unipolar sensingmode) to an amplifier 14A. The amplifier 14A in turn feeds an atrialsensing circuit or sensor and other signal processing circuitry labeledas 15A in FIG. 1. The battery 19 provides power to the amplifier 14A andthe sensor-signal processing circuitry 15A as well as the chargingcircuit 9A.

The switched capacitor circuit 54A includes first and second samplingswitches 7A1 and 7A2 connected respectively to first (upper) and second(lower) nodes of a sampling capacitor 21A. The respective first andsecond nodes of capacitor 21A are in turn connected to first (upper) andsecond (lower) nodes of filter 13A by way of transfer switches 8A1 and8A2. As illustrated schematically, the controlled sampling switches 7A1and 7A2 are commonly controlled such that they both have the same state(i.e., open or closed) at a particular time. Likewise, the controlledswitches 8A1 and 8A2 are controlled by a common signal.

Before discussing the logic arrangement used to control the variouscontrolled switches of FIG. 1, the basic operation of the switchedcapacitor circuit 54A will be explained. As used herein, a "switchedcapacitor circuit" is a circuit having at least one capacitor and atleast two controlled switches, one of which is connected to a node ofthe capacitor for supplying an input to the capacitor and having theother of the switches connected to the same node of the capacitor foroutputting a stored charge or signal on the capacitor.

Assuming that the atrial channel subsystem 52A is in its sensing state,atrial cardiac signals will be provided on the left nodes of controlledsampling switches 7A1 and 7A2. (Depending upon the state of bipolarsense switch 5A and unipolar sense switch 6A, these cardiac signals mayhave been provided by bipolar sensing or unipolar sensing.) Thecontroller 18 causes the sampling switches 7A1 and 7A2 to simultaneouslyclose shortly after (i.e., break before make) the controller 18 hasopened the switches 8A1 and 8A2. The cardiac signals on the left nodesof the switches 7A1 and 7A2 will by "sampled" by the sampling capacitor21A which stores a charge dependent on the cardiac signals. Thecontroller 18 then opens both of the switches 7A1 and 7A2 simultaneouslyand shortly thereafter (i.e., break before make) simultaneously closesboth transfer switches 8A1 and 8A2 such that the capacitor 21A providesan input signal to the filter 13A, this input signal being dependentupon the original cardiac signals. The filter 13A filters out signals atthe frequency of switching of the switches 7A1, 7A2, 8A1, and 8A2 suchthat the amplifier 14A and the atrial sensing circuit 15A "see" inputswhich track the original cardiac signals on the left nodes of switches7A1 and 7A2.

A highly important feature of the operation of the present isolationcircuit 54A is its ability to avoid cross-talk from the subsystem 52V.For example, if the subsystem 52V is in a pacing state such that astimulating pulse is being applied by capacitor 10V across lines 12V1and 12V2 (bipolar pacing), the isolation circuit 54A prevents thestimulating pulse applied to the ventricular conductor 12V1 from"seeing" a return path to ground by way of lines 12A1 or 12A2 and thesensing circuit 15A. By use of the switched capacitor isolation circuit54A, neither of the lines 12A1 or 12A2 will be susceptable tocross-sensing (sensor 15A sensing part of the ventricular stimulatingpulse). Nor will the ventricular stimulating pulse causecross-stimulation (improper stimulation of atria caused by ventriclestimulating pulse).

An important feature of the present invention is that, except for verylimited conditions discussed in detail below, the atrial sensing circuit15A may continue sensing cardiac signals from the atria even when thestimulating pulse is applied by the ventricular channel subsystem 52V.This is highly advantageous over numerous prior designs wherein thesensor of one channel subsystem had to be switched out of circuit whenthe other channel subsystem was providing a stimulating pulse. In thoseprior designs, the sensing on one channel subsystem simply has to ceasewhen the other channel subsystem is providing a stimulating pulse.

FIG. 2 shows various components within the controller 18 and illustratesa specific arrangement for controlling the controlled switches shown inFIG. 1. For ease of illustration, only the switch 20 is shown in FIG. 2,which also illustrates the control line SW20 for the switch 20. However,the numerous lines extending at the right side of FIG. 2 are connectedto one or more of the switch control lines with the correspondingnumber. For example, the top line is labeled SW6A and extends to thecontrol line of switch 6A, whereas the 6th line from the top is labeledboth SW7A1 and SW7A2 to indicate that this control line extends to thecontrol terminal of both switches 7A1 and 7A2.

The primary components of the controller 18 (FIG. 1) which are shown inFIG. 2 are the pacemaker microcontroller 30, master oscillator 31,frequency divider 32, and the break before make non-overlapping twophase clock generator 33. The microcontroller 30 is operable to receivesensing signals from 15A and 15V which are used in known fashion tocontrol the timing of an "APACE" signal and a "VPACE" signal which areused to respectively cause emission of stimulating pulses from theatrial and ventricular stimulator capacitors 10A and 10V (FIG. 1).Additionally, the microcontroller 30 puts out an "A PULSE WIDTH CONTROL"signal and a "V PULSE WIDTH CONTROL" signal to control the width of therespective atrial and ventricular stimulating pulses in known fashion.

Of more central importance to the present invention than theabove-discussed outputs of the microcontroller 30 are signals labeled as"A BIPOLAR SENSE */ A UNIPOLAR SENSE" signal (hereafter "A SENSE"signal), "A BIPOLAR PACE */ A UNIPOLAR PACE" (hereafter "AMPACE"), "VBIPOLAR SENSE */ V UNIPOLAR SENSE" (hereafter "V SENSE"), and "V BIPOLARPACE */ V UNIPOLAR PACE" (hereafter "VMPACE"). The asterisks in thesignal name indicates that that function is asserted when the line is atlogic low. For example, the A SENSE signal causes bipolar atrial sensingwhen it is at logic low and atrial unipolar sensing when it is at logichigh.

A quite important feature of the programmability of the presentinvention is that the different channel subsystems may be independentlyprogrammed for pacing and sensing.

As the controller of FIG. 2 is essentially symmetric about a horizontalcenter line, the logic arrangement will be discussed with reference tothe atrial or upper logic gates. It will be readily understood that theventricular or lower gates operated in corresponding fashion. Forconvenience, the logic gates and related elements are labeled an "A"suffix for atrial components and a "V" suffix for ventricular channelsubsystems.

The A SENSE signal maintains switches 6A and 5A in opposite states.Specifically, if the A SENSE signal is high, the control line SW6A willbe high, thereby closing switch 6A (refer back momentarily to FIG. 1).At the same time, the inverter 34A causes the control line SW5A ofswitch of 5A to be low such that switch 5A will be opened. When the ASENSE signal is low, the switch 6A will be open and the switch 5A willbe closed. With reference to FIG. 1, it will be seen that the positionof these switches 5A and 6A will determine whether the sensing circuit15A is sensing cardiac signals based upon the potential differencebetween 12A1 and 12A2 (bipolar sensing) or based upon the differencebetween conductor 12A1 and the pacemaker can (unipolar sensing).

The APACE signal causes the atrial pace one shot (monostablemultivibrator) 35A to output a pulse whose width is determined by thepulse width control line. The pulse is output from one shot 35A to thecontrol terminal for switch 1A, thereby closing switch 1A. Assuming thatthe atrial channel is in a bipolar pacing mode (AMPACE is low), theinversion of AMPACE output by inverter 55A will be high, thereby causingAND gate 37A to output a high signal. This high signal output by gate37A will cause switch 2A to simultaneosly close with the closing ofswitch 1A such that capacitor 10A (FIG. 1) will discharge across lines12A1 and 12A2 to provide a bipolar stimulating pulse. Additionally, theoutput of inverter 55A is fed into an AND gate 39A, the output of whichcontrols switch 3A. The other input of AND gate 39A is the output of adump one shot 36A which triggers at the end of the pulse put out by oneshot 35A. Upon occurrence of the pulse out of one shot 36A and assumingthat the AMPACE signal is low corresponding to bipolar pacing, theoutput of gate 39A will be high such that switch 3A closes to dissipateany polarization along lines 12A1 and 12A2 (FIG. 1). The switches 1A and2A would of course be open after cessation of the pulse out of one shot35A. A NOR gate 40A is connected to the outputs of one shots 35A and 36Ato provide an output which is high only when neither of the one shots isputting out a pulse. The output of this gate 40A is fed into a AND gate41A. The output of gate 41A controls the switches 7A1 and 7A2 such thatthese switches are maintained opened when the stimulator of the atrialchannel subsystem is outputting a stimulating pulse and until thedissipation switch 3A is opened. In similiar fashion to a person closingtheir eyes to avoid being temporarily blinded by a bright flash,maintaining the switches SW7A1 and SW7A2 open during pulsing by the samechannel prevents the sensing circuit 15A from being temporarily"blinded" or saturated by the signal by the stimulating capacitor 10A.With one execption noted in detail below, the sensing circuit 15A maycontinue sensing even when the stimulating capacitor 10V of theventricular channel subsystem 52V is outputting a stimulating pulse.

The operation of switch 20 depends upon conditions in both the atrialchannel subsystem and the ventricular channel subsystem. Specifically,the control line SW20 of switch 20 is operated by the OR gates 42A and42V and the AND gate 43. If both channels are in bipolar pacing mode,the switch 20 will be closed only when neither channel is in its pacingstate (i.e., neither of the pace one shots 35A and 35V is outputting apulse) and neither of the channels is in a dissipation state (i.e.,neither dump one shot 36A or dump one shot 36V is outputting a pulse).Conversely, upon occurrence an output pulse from any of the one shots35A, 36A, 35V, or 36V, this will cause the output of gate 42A or gate42V (depending upon which one shot is outputting a pulse) to assume alow state such that the output gate 43 is made low. This in turn opensthe switch 20.

In the case where the AMPACE signal is high indicating that the atrialchannel will be in unipolar pacing mode, the supplying of this highAPACE signal to one of the inputs of gate 42A will cause its output toremain high. If the VMPACE signal is likewise high, the output of gate42V will be maintained high and by operation of gate 43, the switch 20will be maintained closed. On the other hand, if the atrial channel isin unipolar pacing mode and the ventricular channel is in bipolar pacingmode (VMPACE is low), the gates 40V, 42V, and 43 will cause switch 20 toclose only during the absense of output pulses from the one shots 35Vand 36V.

Again assuming that our atrial channel portion of the controller is in abipolar pacing mode, the switches 7A1 and 7A2 are commonly controlled byan output of gate 41A to open and close at a frequency of 10 kilohertzcorresponding to the output C2 of the two phase clock generator 33.Clock output C1 from the clock generator 33 is non-overlapping with theoutput C2 and is used to control the switches 8A1 and 8A2.

Assuming that the atrial channel is set for unipolar pacing (AMPACE ishigh), the operation of the logic gates will now be discussed. Inverter55A and gate 37A will cause switch 2A to remain open even when the oneshot 35A is outputting a pulse. Switch 1A would be closed when the oneshot 35A is outputting a pulse and switch 20 would be closed such thatthe stimulating pulse from stimulator capacitor 10A is placed acrossline 12A1 and the pacemaker can. During the dissipation state (i.e., oneshot 36A outputting a pulse), gate 39A by virtue of its input frominverter 55A prevents the switch 3A from closing. Instead, gate 38Acauses switch 4A to close for dissipation when the atrial channel is inthe unipolar pacing mode.

With AMPACE high corresponding to the unipolar pacing mode for theatrial channel, the output of inverter 55A will be low. This output isfed as the upper input to OR gate 44A. As 44A receives its other inputfrom the output of gate 40A, the output of gate 44A will be high onlyupon the non-occurrence of pulses from one shots 35A and 36A. The outputof 44A is in turn fed as an input to OR gate 45V, the output of whichwill remain high at least during the non-occurrence of pulses from oneshots 35A and 36A. As the output of gate 45V is fed into gate 41V, theswitches 7V1 and 7V2 will be opened and closed in accord with the clockoutput C2. Moreover, if the V SENSE signal is low corresponding tobipolar sensing on the ventricular channel, the output of inverter 34Vwill remain high and the output of gate 45V will in turn remain highsuch that the operation of switches 7V1 and 7V2 will be independent ofthe occurrence of a stimulating pulse on the atrial channel. On theother hand, the gates 44A, 45V, and 41V will maintain the switches 7V1and 7V2 open upon the occurrence of an output from one shot 35A or oneshot 36A provided the atrial channel is in a unipolar pacing mode andthe ventricular channel in a unipolar sensing mode. By maintaining theswitches 7V1 and 7V2 open under these very limited circumstances, onewill avoid creating a return current flow through the path of 12V1, 7V1,21V, 7V2, 6V, and 20. In other words, the sensing circuit 15V maynormally continue sensing cardiac signals by virtue of the samplingswitches (7V1 and 7V2) and the transfer switches (8V1 and 8V2) exceptunder the very limited circumstances where the ventricular channel is ina unipolar sensing mode and the atrial channel is in a unipolar pacingmode. Under those very limited circumstances, the sampling switches 7V1and 7V2 are maintained open during the pacing state (i.e., when thestimulating pulse is actually being applied) and the dissipation state(i.e., when the polarization voltage in the atrial channel is beingdischarged) of the atrial channel.

Though the operation of the gates 44A, 45V and 41V have been describedas preventing return current path when the ventricular channel issensing in a unipolar mode and the atrial channel is pacing in unipolarfashion, it will be readily appreciated that the gates 44V, 45A, and 41Aoperate in identical fashion to prevent the establishment of a returncurrent path when the atrial channel is in unipolar sensing mode and theventricular channel is in unipolar pacing mode.

The master oscillator 31 provides a 40 kilohertz signal to both themicrocontroller 30 and a frequency divider 32. The microcontroller 30uses a resident program to determine when stimulating pulses should beemitted based upon the sensed cardiac signals in accord with any ofnumerous known programs or procedures. The frequency divider 32 suppliesa 10 kilohertz signal to two phase clock generator 33 which in turnoperates the switches 7A1, 7A2, 8A1, 8A2, 7V1, 7V2, 8V1, and 8V2.

Turning now to FIGS. 3 and 4, a simplified embodiment of the presentinvention will be discussed. In particular, FIG. 3 shows a simplifiedpacer 150 which is limited to bipolar pacing and bipolar sensing. As theoperation of this embodiment is identical to the described operation ofbipolar sensing and bipolar pacing of the FIG. 1 embodiment, theindividual component parts have been labeled with the same numerals asthe corresponding part in the FIG. 1 embodiment. However, the pacer 150of FIG. 3 is shown as including an atrial channel subsystem 152A and aventricular channel subsystem 152V in light of the differences (i.e.,fewer components) in subsystem 152A and 152V. In similiar fashion, thevarious components of FIG. 4 have been labeled with the same numerals asthe corresponding components of the FIG. 2 control arrangement.

The operation of the embodiments of FIGS. 3 and 4 is identical to thatof the embodiments of FIGS. 1 and 2 under bipolar sensing and pacing.Accordingly, it may simply be noted that FIGS. 3 and 4 are of simplierconstruction and do not provide the programmability to variousoperational modes as with the embodiments of FIGS. 1 and 2.

Although the embodiment shown in FIGS. 1 and 2 is highly advantageousover prior designs in that one channel may maintain its sensing evenwhen the other channel is outputting stimulating except under the verylimited circumstances of unipolar pacing in one channel and unipolarsensing in the other channel, it will be readily appreciated that theembodiment shown in FIGS. 3 and 4 may maintain cardiac sensing on onechannel completely independently of the emission of a stimulating pulseon the other channnel. That is, because the embodiments of FIGS. 3 and 4does not use unipolar sensing and unipolar pacing, one need not includelogic gates to avoid closure of switches 7A1, 7A2, 7V1, or 7V2 underunipolar pacing or unipolar sensing conditions.

FIG. 5 shows an embodiment of a suitable filter 13A according to thepresent invention. Although the filter 13A (or 13V) as used with theswitched capacitor isolation circuit 54A could be as simple as a singlecapacitor connecting the two right nodes (refer back momemtarily toFIG. 1) of switches 8A1 and 8A2, the filter 13A includes a capacitor 202across those two nodes and series capacitors 204 and 206 with a resistor208 extending from the common node of the seriesed capacitors to theground. The illustrated filter 13A together with a switching frequencyof 10 kilohertz for the sampling and transfer switches (FIG. 1 only)constitute a band pass filter with a center frequency of about 40 hertz.

FIG. 6 shows a simplified diagram illustrating how the microcontroller30 may by programmed. The present invention may use known componentssuch as a transceive coil 210, digital decoder 212, and error detect 214to input signals to the microcontroller 30. The microcontroller 30,which would include memory storage and a programmed microprocessor, maythus be changed to operate in different modes, each of the channelsbeing operable in one of four different modes independently of the otherchannel. Specifically, the four modes are:

I. unipolar pacing and unipolar sensing,

II. unipolar pacing and bipolar sensing,

III. bipolar pacing and unipolar sensing, and

IV. bipolar pacing and bipolar sensing.

The transceive coil 210 also may be connected to the microcontroller 30via transmitter 215 and digital encoder 217 to transmit informationabout pacer operation inductively to an instrument (not shown) placedoutside the human body. The details of transmission of information toand from the pacer while disposed in the human body are not central tothe present invention and are therefore described only generally.

FIG. 7 shows a possible configuration for a two phase clock generator 33(refer back momemtarily to FIGS. 2 or 4). As shown, the clock generator33 receives the 10 kilohertz input clock and uses NAND gates 218 and 220together with inverters 222, 224 and 226 in the configuration as shownsuch that the output clocks C1 and C2 will be non-overlapping 10kilohertz signals.

Although various specific constructions and component values have beendescribed herein, it is to be understood that these are for illustrativepurposes only. Various modifications and adaptations will be readilyapparent to those of ordinary skill in the art. Accordingly, the scopeof the present invention should by determined by reference to claimsappended hereto.

What is claimed is:
 1. A cardiac pacer comprising:a ventricular channelsubsystem; an atrial channel subsystem; at least one of said subsystemshaving a stimulator connected to a pacing/sensing lead for supplyingcardiac stimulating pulses, and the other of said subsystems havingsensing circuit means for sensing cardiac signals received from anotherpacing/sensing lead; said other of said subsystems including a switchedcapacitor isolation circuit connected between said anotherpacing/sensing lead and said sensing circuit means; said switchedcapacitor isolation circuit serving to avoid any return current path tosaid sensing circuit means of said other of said subsystems forstimulating pulses from said stimulator, and said switched capacitorisolation circuit including:a sampling capacitor having first and secondnodes; controlled first and second sampling switches connected betweensaid another pacing/sensing lead and said first and second samplingcapacitor nodes, respectively, for controllably applying cardiac signalsacross said sampling capacitor; a filter having first and second inputnodes and an output connected to said sensing circuit means of saidother of said subsystem; and controlled first and second transferswitches connected between said first and second sampling capacitornodes and said first and second filter input nodes, respectively, forcontrollably, connecting said sampling capacitor to said filter; andmeans for controlling said first and second sampling switches and saidfirst and second transfer switches such that said first and secondsampling switches are both open whenever either one of said first andsecond transfer switches is closed and such that said first and secondtransfer switches are both open whenever either one of said first andsecond sampling switches is closed.
 2. The cardiac pacer of claim 1wherein said one of said subsystems also includes a sensing circuitmeans for sensing cardiac signals.
 3. The cardiac pacer of claim 2wherein said one of said subsystems further comprises:a bipolarpacing-sensing lead having first and second conducting wires, andcontrolled first and second power switches switchable to allow power toflow from said stimulator to said lead.
 4. The cardiac pacer of claim 3wherein said one of said subsystems is disposable in a sensing state andsaid controller is operable to change said one of said subsystemsbetween said sensing state and a pacing state in which said first andsecond power switches are closed to allow power flow from saidstimulator to said lead, and wherein said first subsystem furthercomprises a dissipation switch for shorting said first and secondconducting wires together, said controller serving to close saiddissipation switch after said pacing state and before said sensingstate.
 5. The cardiac pacer of claim 2 wherein said controller changesthe state of both of said first and second sampling switchessimultaneously and changes the state of both of said first and secondtransfer switches simultaneously.
 6. The cardiac pacer of claim 2wherein said other of said subsystems has a stimulator for supplyingcardiac stimulating pulses, and said one of said subsystems has anotherswitched capacitor isolation circuit provides no return current path tosaid sensing circuit means of said one of said subsystems forstimulating pulses from the stimulator of said one of said subsystemseven when the sensing circuit of said one of said subsystems is sensingcardiac signals,said another switched capacitor isolation circuitincluding the following components connected and operable by said meansfor controlling in like fashion to the corresponding component of saidother of said subsystems:a sampling capacitor; controlled first andsecond sampling switches; a filter; and controlled first and secondtransfer switches.
 7. The cardiac pacer of claim 6 wherein saidcontroller is operable to dispose each of said subsystems in thefollowing modes:I. unipolar pacing and unipolar sensing, II. unipolarpacing and bipolar sensing, III. bipolar pacing and unipolar sensing,and IV. bipolar pacing and bipolar sensing.
 8. The cardiac pacer ofclaim 7 wherein said means for controlling is operable to set the modeof each subsystem independent of the mode of the other subsystem.
 9. Thecardiac pacer of claim 7 wherein said means for controlling maintainsthe first and second sampling switches of a subsystem open when thestimulator of that subsystem is supplying cardiac stimulating pulses.10. The cardiac pacer of claim 7 wherein, in at least some modes, saidmeans for controlling allows the sensing circuit of either one of thesubsystems to continue sensing cardiac signals even when the stimulatorof the other of the subsystems is supplying stimulating pulses and withsaid switched capacitor circuit of the one subsystem preventing thestimulating pulses of the stimulator of the other subsystem from findinga return current path in the one subsystem.
 11. The cardiac pacer ofclaim 1 wherein said means for controlling allows the sensing circuit ofthe other subsystem to continue sensing cardiac signals even when thestimulator of the one subsystem is supplying stimulating pulses and withsaid switched capacitor isolation circuit of the other subsystempreventing the stimulating pulses of the stimulator of the one subsystemfrom finding a return current path to the other subsystem.
 12. A cardiacpacer comprising:a ventricular channel subsystem and an atrial channelsubsystem, each subsystem having:a lead for receiving cardiac signals; asensing circuit means for sensing cardiac signals; a stimulator forsupplying cardiac stimulating pulses; and a switched capacitor isolationcircuit connected between the lead and the sensing circuit means, saidswitched capacitor isolation circuit serving to avoid any return currentpath for stimulating pulses from the stimulator of the other subsystem;and means for controlling each of the switched capacitor isolationcircuits.
 13. The cardiac pacer of claim 12 wherein said means forcontrolling is operable to allow the sensing circuit of either one ofthe subsystems to continue sensing cardiac signals even when thestimulator of the other of the subsystems is supplying stimulatingpulses and with the switched capacitor circuit of the one subsystempreventing the stimulating pulses of the stimulator of the othersubsystem from finding a return current path in the one subsystem. 14.The cardiac pacer of claim 12 wherein said controller is operable todispose each of said subsystems in the following modes:I. unipolarpacing and unipolar sensing, II. unipolar pacing and bipolar sensing,III. bipolar pacing and unipolar sensing, and IV. bipolar pacing andbipolar sensing.
 15. The cardiac pacer of claim 14 wherein thecontroller is operable to set the mode of each subsystem independent ofthe mode of the other subsystem.
 16. The cardiac pacer of claim 12wherein each switched capacitor isolation circuit includes:a samplingcapacitor having first and second nodes; controlled first and secondsampling switches connected between the lead and said first and secondsampling capacitor nodes, respectively, for controllably applyingcardiac signals across said sampling capacitor; a filter having firstand second input nodes and an output connected to said sensing circuitmeans; and controlled first and second transfer switches connectedbetween said first and second filter input nodes, respectively, forcontrollably, connecting said sampling capacitor to said filter; saidcontroller being connected so as to control said first and secondsampling switches of each subsystem and the first and second transferswitches of each subsystem such that the first and second samplingswitches of a subsystem are both open whenever either of said first andsecond transfer switches of that subsystem are closed and such that thefirst and second transfer switches of a subsystem are both open whenevereither of the first and second sampling switches of that subsystem areclosed.
 17. A cardiac pacer comprising, a housing; a controller; aventricular channel subsystem; and an atrial channel subsystem; eachsubsystem including:two pacing-sensing conductors; a stimulatorconnected to said two pacing-sensing conductors for supplying cardiacstimulating pulses; an isolation circuit connected to said twopacing-sensing conductors; and a sensing circuit connected to saidisolation circuit for receiving sensed cardiac signals from said twopacing-sensing conductors by way of said isolation circuit; andwhereinthe isolation circuit of each subsystem isolates the sensing circuit ofthat subsystem against return current from stimulating pulses from thestimulator of the other subsystem; and further comprising a controlleroperable to dispose each of the subsystems in the following modes,independent of the mode of the other subsystem: I. unipolar pacing andunipolar sensing; II. unipolar pacing and bipolar sensing; III. bipolarpacing and unipolar sensing; and IV. bipolar pacing and bipolar sensing.18. The cardiac pacer of claim 17 wherein, in at least some modes, theisolation circuit of either one of the subsystems allows the sensingcircuit of the one subsystem to continue sensing cardiac signals evenwhen the stimulator of the other of the subsystems is supplyingstimulating pulses and with the isolation circuit of the one subsystempreventing the stimulating pulses of the stimulator of the othersubsystem from finding a return current path in the one subsystem. 19.The cardiac pacer of claim 18 wherein, in said at least some modes, thesensitivity of the sensing circuit in each subsystem is independent ofthe occurrence of stimulating pulses from the stimulator of the othersubsystem.
 20. The cardiac pacer of claim 19 wherein said at least somemodes includes all modes except where the one subsystem is usingunipolar sensing and the other subsystem is using unipolar pacing. 21.The cardiac pacer of claim 18 wherein each of the isolation circuits isa switched capacitor circuit.
 22. The cardiac pacer of claim 21 whereineach of the switched capacitor circuits includes:a sampling capacitorhaving first and second nodes and operable to sample cardiac signals;controlled first and second sampling switches connected respectively tosaid first and second nodes of said sampling capacitor and operable forswitchably applying cardiac signals across said sampling capacitor; afilter having first and second nodes and operable to store cardiacsignals and to supply cardiac signals to the corresponding sensingcircuit; and controlled first and second transfer switches, said firsttransfer switch switchably connecting said first node of said samplingcapacitor to said first node of said filter, said second transfer switchswitchably connecting said second node of said sampling capacitor tosaid second node of said filter; andwherein said controller is operableto control the first and second sampling switches of each subsystem andthe first and second transfer switches of each subsystem such that thefirst and second sampling switches of a subsystem are both open whenevereither of said first and second transfer switches of that subsystem areclosed and such that the first and second transfer switches of asubsystem are both open whenever either of the first and second samplingswitches of the subsystem are closed.
 23. The cardiac pacer of claim 22wherein said controller maintains the first and second sampling switchesof a subsystem open when the stimulator of that subsystem is supplyingcardiac stimulating pulses.
 24. The cardiac pacer of claim 21 whereinsaid controller includes a phase clock generator with non-overlappingfirst and second outputs, the first output controlling said first andsecond sampling switches in said atrial channel subsystems and saidfirst and second sampling switches in said ventricular channelsubsystem, and the second output controlling said first and secondtransfer switches in said ventricular channel subsystem and said firstand second transfer switches in said atrial channel subsystem.